As one of method for increasing data transmission efficiency in a wireless communication system, there is a multi-input multi-output (MIMO) technology. A MIMO scheme can be classified into a single user-MIMO (SU-MIMO) scheme and a multi user-MIMO (MU-MIMO) scheme according to whether data can be simultaneously transmitted using the same band when the data is transmitted to a plurality of users.
A MIMO system can be divided into an open loop mode in which an eNB performs communication without knowing a channel state and a closed loop mode in which an eNB performs communication with reference to channel information fed back from a user equipment (UE). In general, a mainly used scheme is a closed loop scheme for achieving approximate theoretical transmission capacity by applying an independent modulation and coding scheme according to a channel state for each transmission antenna.
In the closed-loop type MIMO system, a UE can use a codebook in order to transmit channel information to an eNB. Codewords included in a codebook indicate different channel states for respective channels between the eNB and the UE. The UE estimates a channel using a reference signal received from the eNB, selects a codeword corresponding to the estimated channel, and then feeds back an index of the codeword to the eNB to notify the eNB of the channel state. When the eNB performs beamforming using each column vector of the codebook as a beamforming vector, the UE calculates the quality of a downlink channel and generates a downlink channel quality indictor. Then the UE feeds back a position of a column vector corresponding to a best downlink channel quality indictor and a downlink channel quality indictor corresponding to the position to the eNB.
For convenience of description, when an ith quantization vector present in a codebook is defined as qi irrespective of a rank, a quantization vector can be selected according to the following equation.
                              q          k          *                =                  arg          ⁢                                          ⁢                                    max              i                        ⁢                                                                          H                  k                                ⁢                                  q                  i                                                                                                      [                  Equation          ⁢                                          ⁢          1                ]            
Here, arg max f(x) indicates x for making f(x) with a maximum value. Hk indicates a channel vector of a kth UE. Each UE transmits an optimum quantization vector index via the above procedure.